Recent advances in the manipulation of text and graphics at the pre-press stage have created a need for lithographic plates suitable for exposure by low-energy devices such as laser scanners and projection cameras. These devices frequently emit actinic radiation over a narrow wavelength range, and accordingly, the sensitivity of a useful lithographic plate must not only be very high, so as to reduce scan times and exposure times to realistic levels, but also must also be sensitized to match the output of the exposure device.
Apart from the sensitivity requirements, various other attributes are necessary for such a resist to function as a high-speed positive plate. These include processing latitude, resolution, and press-life, all of which may be interrelated. Press-life is governed by (among other factors) the durability and adhesion of the resin which forms the image, hence the widespread use of phenolic resins coated on anodized aluminum, especially as these resins may frequently be post-baked (following development) to further enhance the image durability.
With regard to processing latitude, the ideal composition would allow the exposed areas of the coating to dissolve rapidly and completely in the developer, leaving the unexposed areas unaffected. In practice, this is rarely, if ever, achieved, largely as a result of the use of phenolic resins which are generally attacked, to some extent, even when not further solubilized by the alkaline developer. Part of the skill of positive print making lies in adjusting the development conditions so that attack on the unexposed areas is minimized while maintaining efficient wash-out of the exposed areas. Parameters that may be varied include time of development, temperature, and pH of the developer. At one extreme, if the development conditions are too aggressive, so much coating is lost from the resin image that press-life is reduced. At the other extreme, too mild development conditions lead to incomplete dissolution of the exposed areas causing background images to be made on press.
Over or under-development also affects the resolution obtained. High quality color printing requires the reproduction of half-tone images with dots in the range of about 3% to 97% obtained via a 150 line screen, or finer. Therefore, the development conditions must allow the shadow dots to be opened without excessive attack on the highlight dots which might cause them to be removed altogether. A convenient rule of thumb states that acceptable results are obtained if, under development conditions that produce complete dissolution of the exposed areas, coating weight loss from the unexposed areas is in the range of 5% to 15%.
From the point of view of convenience to the plate maker, it is desirable that the plate should possess sufficient development latitude to allow a range of developer solutions to be used, rather than being restricted to tightly controlled conditions.
Positive-acting resists generally comprise a resin binder, such as a phenolic resin, which is readily soluble in the developer solution, e.g., aqueous alkali. One or more light-sensitive compounds are added to the resin which have the property of retarding or totally suppressing the dissolution of the resin in the developer. By exposure to light of the appropriate wavelength and intensity, the light-sensitive compounds are converted to other chemical species which exert no insolubilizing action or may even assist solubilization. By this means, the original solubility of the resin is restored. This process is known as photosolubilization.
Virtually all the commercially available positive-acting resin-on-metal plates combine a phenolic resin with a light-sensitive diazo oxide as a solubility inhibitor. This technology is described, for example, by Brinckman et al. in Unconventional Imaging Processes; Focal Press, New York, 1987; pp 65-67. It has low sensitivity (ca. 10.sup.6 erg/cm.sup.2, or 100 mJ/cm.sup.2) and is not believed to be sensitized beyond its intrinsic sensitivity range centered in the near-ultraviolet.
U.S. Pat. No. 4,708,925 (Newman) discloses the combination of an onium, preferably iodonium, salt and a phenolic resin as a positive-acting photoresist. This represents a considerable advance both in terms of speed (ca. 10.sup.5 erg/cm.sup.2 or 10 mJ/cm.sup.2) and also in the capability to be sensitized throughout the entire visible spectrum and into the infrared. Nevertheless, further increases in speed are desirable, and the processing latitude of this system is insufficient for many applications.
The speed attainable from two-component resists of this type is limited by the lack of any amplification of the initial photochemical event. Negative-acting photopolymer resists achieve higher sensitivities by virtue of the fact that a single photon can initiate a chain reaction that produces tens or hundreds of insolubilizing bonds. Therefore, in recent years a number of photocatalytic solubilizing systems have been proposed for high speed positive-acting photoresists, each exploiting reactions catalyzed by photo-generated acid. One such system includes t-alkyl carbonate polymers as described by Willson (e.g., Jiang, Y.; Frechet, J. M. J.; Willson, C. G. Polymer Bulletin 1987, 17, 1-6; Frechet, J. M. J.; Bouchard, F.; Houlihan, F. M.; Kyyczka, B.; Eichler, E.; Clecak, N.; Willson, C. G. J. Imag. Sci. 1986, 30, 59-64. These systems contain t-alkyl carbonate groups either as pendant groups or within the polymer backbone, which cleave catalytically in the presence of photo-generated acid, causing a marked polarity change (release of polar groups) or depolymerization to soluble or volatile fragments. These materials are designed specifically for use as semiconductor masks, and are formulated simply with an onium salt as acid generator; i.e., they contain no additional phenolic resin as binder. Hence, they are not suitable to function as printing plates with a realistic run-length.
There are various known resist compositions which contain three essential ingredients, namely a resin binder, a photo-acid source, and a cleavable compound.
Research Disclosure No. 27,721 (May 1987) discloses the combination of the carbonate polymers described above with an onium salt and a resin binder which is an aromatic vinyl polymer (e.g., polystyrene), not a phenolic resin. The examples cited therein do not indicate how the plates are developed and the quoted sensitivity (Example 2) is 150 mJ/cm.sup.2 (i.e., 1.5.times.10.sup.6 erg/cm.sup.2), which is very low.
Eur. Appl. 249,139 (Crivello) discloses the combination of a phenolic resin, an onium salt, and an acid-sensitive compound, where the last named is preferably a t-butyl ester or a t-butyl carbonate. The formulations are useful in the production of integrated circuits and the like. The highest sensitivities quoted in the visible or near-ultraviolet are 3 to 4.times.10.sup.5 erg/cm.sup.2 (30 to 40 mJ/cm.sup.2).
Ger. Patent 3,601,264 (Aoai) and Jap. Kokai No. 62/059,949 disclose the combination of iodonium salt, phenolic resin binder, and respectively, a silyl ether compound, and a silyl ureido compound. No information is given on resolution or development latitude. Formulations containing the silyl ether compound are shown to be up to ten times faster than a diazo oxide control.
U.S. Pat. Nos. 3,915,704 (Limburg); 3,915,706 (Limburg); 3,917,483 (Limburg); 3,932,514 (Halcour); 4,247,611 (Buhr); 4,248,957 (Sander); 4,250,247 (Buhr); and 4,311,782 (Buhr) disclose phenolic resin binder, triazine derivatives and diazonium salts as acid sources, and respectively, polyacetals/polyketals, enol ethers, N-acyliminocarbonate, and polymeric orthoester cleavable compounds. According to the examples therein, offset plates produced by these methods require 20 sec exposure to a 5 kW lamp at 110 cm (15 sec in the case of poly(orthoesters)). There is no indication of the processing latitude or resolution.
Orthoesters have been used as acid-sensitive solubility inhibitors (e.g., see Ger. Appl. 2,610,842 (Roth)).
U.S. Pat. Nos. 4,101,323 (Roth) and 4,806,448 (Roth), and Research Disclosures No. 27,331 (Jan. 1987) disclose the combination of phenolic resin with an onium salt, together with cyclic acetals or ketals of .beta.-ketocarbonyl compounds or poly(phthalaldehyde). Additionally, the Research Disclosure indicates that other cleavable polymeric compounds may be used as a third component, giving improved speed and process latitude, but no specific details are given. Offset plates disclosed in the cited U.S. patents required 60 sec exposure to a 5 kW source at 65 cm to clear 3 steps on a 0.15 increment gray scale. There is no teaching as to resolution or development latitude.
U.S. Pat. No. 3,779,778 (Bonham) discloses the combination of phenolic resin, acid source, and a variety of cleavable compounds, including pyranyl ethers of bisphenols. The acid source is a polyhalogen compound, preferably a polyhalogenated triazine derivative.
None of these prior art publications teach the importance of the developer resistance of the exposed material, or of how it may be maximized. The publications are only concerned with the photosolubility produced by the photocatalytic degradation of certain compounds.